Sat navs seem to have a slight lag. If you are going a constant speed they very accurate. There seems to some doubt whether the algorithm takes into account hills. My speedo reads 60 when sat nav says 55
When I was over there I would drive pass speed cameras at the roads legal limit according to the TomTom usually the speedo read 5mph or so higher, I have never had a ticket. I do not take responsibility for any tickets you may get if you try this.
I go by the satnav - speedo in KMH and is around 10kmh out. Never had a ticket yet even when I've been on roadworks limited to 50, with the average speed cameras I'd rather go by GPS than speedo.
